why do we need ArrayBuffer

I am trying to understand ArrayBuffer.

It seems like that if we use uint8Array directly, we get everything we need(typed array with fixed type).

We can do something like this:

var a = new Uint8Array(2)
a[0] = 10
a[1] = 20;

we can even do this = console.log(a.buffer) which means we still have a buffer meaning that Uint8Array itself is a buffer.

So why do we need ArrayBuffer class then ? What's the actual use case ?